automc: pg LISTEN/NOTIFY route source + HTTP waker

Adds opt-in extension package internal/automc/ that:
- Subscribes to Postgres notifications on a 'servers' table and pushes
  route changes into server.Routes (no file I/O, no fsnotify).
- Provides a WakerFunc that POSTs to a configurable HTTP control plane
  (server-manager) and polls until state=running.

When AUTOMC_DSN is unset, Wire() is a no-op and the binary behaves
exactly like upstream itzg/mc-router. Single patch site in main.go
(import + 4-line call) keeps upstream rebases trivial.

See docs/AUTOMC.md for env vars and the expected DB schema/trigger.

internal/automc/wire.go

@@ -0,0 +1,24 @@
+// Package automc wires automc-specific extensions onto upstream mc-router.
+//
+// All behavior is opt-in via env vars; when AUTOMC_DSN is unset, Wire is a no-op
+// and the binary behaves exactly like upstream itzg/mc-router.
+package automc
+
+import (
+	"context"
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+func Wire(ctx context.Context) error {
+	dsn := os.Getenv("AUTOMC_DSN")
+	if dsn == "" {
+		return nil
+	}
+	waker := newWakerConfig(os.Getenv("AUTOMC_WAKER_URL"), os.Getenv("AUTOMC_WAKER_TOKEN"))
+	s := newSyncer(dsn, waker)
+	go s.run(ctx)
+	logrus.Info("automc: pg route sync started")
+	return nil
+}